Model little cracks on the surface of an object
Well, to alter the physical appearance of a mesh without manually doing it (i.e using a non-destructive workflow), you would want to add certain textures that control the depth of some parts of your mesh, which is what cracks do. 

There are 2 main ways to do this: 

1) Using a displace modifier, which I and most other artists don't recommend unless you want to overlay it with some sort of particle-system, and

2) Giving some bump in the nodes. 



The method we are going to explore is doing this via nodes. I'm not going to explain what they are; although you can read up about them here. 

So, to start off with, what are normal and bump maps? Well, they are both used to create a fake perception of depth. This is imperative. They do not displace any actual geometry, but make it seem as if they had in render mode. For the purpose of this answer, I'm going to use bump maps, which convert grayscale values to depth, simply because they're easier to create procedurally. 



Bump maps take textures like this: 



and then convert them into depth. Generally, brighter values mean more coming-outwards and vice-versa, although it can be inverted. Let's get started then!



In the node editor, in the normal area of the default Principled BSDF material of the material applied to your object, add-in and plug-in a Bump node. Then, add a Voronoi Texture. This is what will control our bump values. 

Change the 3D to 4D, and then choose Distance To Edge. Turn the scale down to something you like, then connect the Voronoi Texture to the height input of the Bump node. Check invert on the Bump node. Great! You have cracks now! Although they don't look like how you expected, so add in a ColorRamp node between the Voronoi texture and Bump node. Enable the Node Wrangler addon, and then Ctrl Shift Click the ColorRamp node to view it. Move the sliders around to your liking (the closer white is to the left, the less bump in the area between the cracks, and the more right the black slider is, the more outlined the cracks are). 

This is where we are right now: 



Here, you can end. You have achieved making cracks. However, you could still do new things, such as add colour to the cracks and stuff! 



Or you could add image textures instead of flat boring colours! Go crazy. 



(Documentation?) links where you can learn more about these techniques: 

Bump Node

Normal Mapping Node

Introduction to Nodes... 

There's a lot. I suggest you start with those however. Hope this answer helped you!